Existing product pumps rely on gravity to transfer the product from a hopper, or the like, into a pump chamber. However, gravity flow limits a pump to use with free flowing products. Also, product pumps in the past were unable to discharge into a vessel that gave a high back pressure.
Small quick cycling product pumps were frequently used as a means of introducing a product into a pneumatic conveying line. Each successive cycle of the pump pushed a new charge of product into the conveying line and the air trapped between each charge pushed the previously-loaded charge further along the conveying line. These devices are normally cycled with electric or pneumatic controls incorporating adjustable timing devices. The timers are arranged to vary the time of each function within the cycle so that the system can be adjusted to meet varying field conditions. The cycle starts with the product inlet valve open and product flows by gravity from the hopper, or the like, into the product pump. After a short fill time, the product inlet valve is closed and the air pressure is applied to the pump chamber to force the product charge from the pump into the conveying line. After the air flow is shut off, air pressure remains in the product pump and in the conveying line. Some of this air will dissipate through the product charge and out the discharge of the conveying system providing a delay time is employed between the compressed air shut off and the opening of the product inlet valve. In actual practice, a delay time is selected that permits some compressed air to bleed off down the pipe with the remaining compressed air blowing up into the product when the inlet valve is opened. Both the time required to bleed off the compressed air and the air blowback reduce the overall delivery rate of the pneumatic conveying system. Additionally, the higher the back pressure at the discharge of the system, the greater the inlet venting problem. Also, a high rate of blowback at the inlet feed point often causes a dust problem. It is further noted that this type of system cannot be used to transport to a pressurized vessel or into a liquid that presents a large hydraulic back pressure.